1. Field of the Invention
The present invention relates to a nitride semiconductor substrate.
2. Related Background Art
In recent years, light emitting diodes (LEDs) and semiconductor lasers (LDs) that use a nitride semiconductor (GaN, AlGaN, InAlGaN) have frequently been used. Meanwhile, in a cladding layer, it is necessary to use AlGaN, etc., which has a higher AlN molar fraction to secure a large difference in band gap in relation to an active layer, in order to attempt to shorten the light-emitting wavelength. Furthermore, in the case of LDs, a cladding layer having a large thickness, the AlN molar fraction of which is still higher, will be required in order to carry out confinement of light.
However, in a nitride semiconductor substrate to form such a device, AlGaN having a high AlN molar fraction has small lattice spacing, and a difference in the lattice spacing between the AlGaN and GaN having relatively large lattice spacing will increase still further. Therefore, if an AlGaN layer is grown directly on a flat GaN layer, a tensile stress is produced in the AlGaN layer due to lattice mismatching between the AlGaN layer and the GaN layer, wherein when the film thickness exceeds a certain critical film thickness, cracks can be generated. Since the GaN layer as a base layer has a number of lattice defects, a number of crystal defects (dislocation) can be also generated in the AlGaN layer grown on the GaN layer.
In order to solve the above-described problems, a technique described in Patent Document 1 (Patent Document 1: Japanese Published Unexamined Patent Application No. 2005-235911) as shown in FIG. 23 and FIG. 24 has been developed. FIG. 23 is a plan view of a nitride semiconductor substrate 1′, and FIG. 24 is a sectional view taken along the arrow XXIV-XXIV of the nitride semiconductor substrate 1′ shown in FIG. 23. The nitride semiconductor substrate 1′ is composed of a substrate 2′, an AIN buffer layer 3′, a first GaN layer 4′, a second GaN layer 6′, and an AlGaN overgrown layer (embedded layer) 7′. The second GaN layer 6′ is selectively grown (Epitaxially Lateral Overgrowth) after striped SiO2 masks 5′ were formed on the first GaN layer 4′, wherein the second GaN layer 6′ having cross-sectionally triangular shapes with minor planes (facets) can be formed. And, since the crystal defects (dislocations) are along the epitaxially growth direction, the dislocations on the second GaN layer 6′ are turned along the lateral direction when the GaN is grown to the lateral direction on the mask 5′ from a portion where the mask 5′ does not exist. On the upper surface of the AlGaN layer 7′ overgrown on the second GaN layer 6′, while dislocations a′ remains at portions corresponding to the top and valley of ridges of the second GaN layer 6′, other portions are brought into a low defect (low dislocation) region R, wherein it is possible to prevent the generation of cracks.